Some radars mounted on vehicles can detect the azimuth angle of a target existing on a road (a deviation angle between a directional line from the radar to the target: hereinafter called a target azimuth angle). For example, “Development Trend of Vehicle Millimeter Wave Radars”, Journal of The Institute of Electronics, Information and Communication Engineers, Vol. 1996, October (pp. 977–pp. 981) describes: (1) a mechanical scan type radar which realizes horizontal scanning of a main beam of a transceiver antenna by mechanically swinging the transceiver antenna; (2) a beam switching type radar which realizes horizontal scanning of a main beam by sequentially switching between transceivers for transmitting beams having different directions; and (3) a monopulse type radar which detects a target azimuth angle in the horizontal plane from a phase difference of reception waves received by two antennas disposed right and left. The radars mounted on vehicles enumerated herein all have as their main object detecting a target on a road, so that only the target azimuth angle in the horizontal plane is detected.